1803 Zwicky

1803 Zwicky, prov. designation:, is a stony Phocaea asteroid and binary system from the inner regions of the asteroid belt, approximately 10 km in diameter. It was discovered on 6 February 1967, by Swiss astronomer Paul Wild at Zimmerwald Observatory near Bern, Switzerland. It was later named after Swiss astronomer Fritz Zwicky. The discovery of a 2.5-kilometer sized companion was announced on 8 March 2021.

Classification and orbit
Zwicky is a member of the Phocaea family (701), an asteroid family with two thousand members, named after their largest member, 25 Phocaea. It orbits the Sun in the inner main-belt at a distance of 1.8–2.9 AU once every 3 years and 7 months (1,316 days; semi-major axis of 2.35 AU). Its orbit has an eccentricity of 0.25 and an inclination of 22° with respect to the ecliptic. It was first identified as at Lowell Observatory in 1931, extending the body's observation arc by 36 years prior to its official discovery observation.

Naming
This minor planet was named after Swiss astronomer Fritz Zwicky (1898–1974), who was a professor at Caltech and a pioneer in many fields, most notably in the study of galaxy clusters and supernovas, in high-energy astrophysics, and in developing jet propulsion for spacecraft and airplanes. He was the first to infer the existence of unseen matter and coined the term Dark matter. The lunar crater Zwicky is also named in his honour. The official was published by the Minor Planet Center on 18 April 1977 (M.P.C. 4156).

Physical characteristics
Zwicky is a bright, stony S-type asteroid, in line with the overall spectral type for members of the Phocaea family.

Lightcurves
In July 2018, a rotational lightcurve of Zwicky was obtained from photometric observations by the TESS-team which gave a rotation period of ($9.2 km$) hours and an amplitude of ($9.934 km$) magnitude (U=2). Observations by Tom Polakis, who also discovered a satellite (see below) determined a very similar period of ($9.61 km$) hours with a brightness variation of ($2.5 km$) (U=2). These more recent result are replacing a previous observation from March 2003, of a fragmentary lightcurve by French amateur astronomer Laurent Bernasconi that gave a tentative period of 27.1 hours and an amplitude of 0.08 (U=1).

Diameter and albedo
According to the surveys carried out by the Japanese Akari satellite and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Zwicky measures between $2.734 h$ and $0.259$ kilometers in diameter, and its surface has an albedo of $0.337$ and $2.734$, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for Phocaea asteroids of 0.23, and calculates a diameter of 10.06 kilometers based on an absolute magnitude of 12.2. The WISE team also published an alternative mean-diameters of ($0.06$) and ($2.733$) kilometers with a corresponding albedo of ($0.105$) and ($9.2$).

Satellite
Photometric observations at the Command Module Observatory by Tom Polakis on 21 February 2021 revealed, that Zwicky has a satellite in its orbit. The moon has a diameter of approximately 2.50 kilometers, or 26% of that of its primary, and an orbital period of 28.46 hours.